The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions.
With the rapid increasing of IP network data services, operators have an increasing demand for transmission capacity, and the capacity of a single fiber in an existing network is gradually approximating the limit of 100 Tbps. Commercial 100G transmission systems has already begun in the year of 2013. How to further increase the transmission capacity on the basis of 100G transmission signals has drawn the attention of equipment manufactures and operators.
In 100G and 100G beyond systems, coherent receiving and digital signal processing (DSP) technologies are adapted at a receive end, and dispersion and polarization mode dispersion accumulated in an entire transmission process can be compensated in the electronic domain. Polarization mode multiplexing and various high order modulation manners, for example, PM-QPSK, PDM-16QAM, and PDM-32QAM, even PDM-64QAM and CO-OFDM, are adapted to reduce a baud rate of a signal. However, high order modulation manners are quite sensitive to the nonlinear effect, and therefore raise a higher requirement for an optical signal-to-noise ratio (OSNR). The introduction of a fiber with a low loss and a large effective area can bring increase in OSNR and decrease in nonlinear effect for a system. When a system with a high power density is used, the nonlinear coefficient, which is defined as n2/Aeff, is a parameter for evaluating system performance due to the nonlinear effect, where n2 is a nonlinear refractive index of a transmission fiber, and Aeff is an effective area of the transmission fiber. The increasing of an effective area of a transmission fiber can reduce the nonlinear effect in the fiber.
Currently, a conventional single-mode fiber for terrestrial transmission system lines has an effective area of only approximately 80 μm2 at the wavelength of 1550 nm. A terrestrial long-distance transmission system has a higher requirement for an effective area of a fiber, where the effective area should be generally larger than 100 μm2. To reduce laying costs, every effort should be made to reduce use of repeaters. In a repeater-free transmission system, for example, a submarine transmission system, an effective area of a transmission fiber is preferably larger than 130 μm2. However, in current designing of a refractivity profile of a fiber with a large effective area, a large effective area is often obtained by increasing a diameter of an optical core layer for transmitting optical signals. Such schemes have some design difficulties. On one hand, a core layer of a fiber and a cladding layer close thereto mainly determine basic performance of the fiber, and occupy a large proportion in manufacture costs of the fiber. If the radial size is designed to be excessively large, the manufacture costs of the fiber is bound to increase, pushing up the price of the fiber, which forms a barrier for wide application of such fibers. On the other hand, compared with a conventional single-mode fiber, an increase in the effective area of the fiber causes deterioration in some other parameters of the fiber: for example, a cutoff wavelength of the fiber increases, and a single-mode state of an optical signal in a transmission waveband in the fiber is difficult to be ensured if the cutoff wavelength is excessively large. In addition, if a refractivity profile of the fiber is not designed appropriately, deterioration in parameters such as a bending performance and a dispersion may also be caused.
Another characteristic of a fiber restricting long-distance large-capacity transmission is attenuation. Currently, an attenuation of conventional G.652.D fiber is generally 0.20 dB/km at 1550 nm wavelength. Laser energy gradually decreases after a long-distance transmission; therefore, a signal needs to be re-amplified by using a repeater. Compared with costs of fibers and cables, costs of repeater related equipment and maintenance occupy more than 70% of those of an entire link system. Therefore, if a low-attenuation or ultralow-attenuation fiber is designed, a transmission distance may be effectively extended, and construction and maintenance costs may be reduced. According to related calculations, if an attenuation of a fiber decreases from 0.20 dB/km to 0.16 dB/km, construction costs of an entire link decreases approximately 30%.
Therefore, it becomes an important subject in the fiber manufacture field to design a fiber with an ultra-low attenuation and a large effective area. U.S. Publication No. US2010/022533 discloses a design of a fiber with a large effective area. To obtain a lower Rayleigh coefficient, a pure-silicon core design is adapted, and there is no germanium and fluorine co-doped in the core layer, and silicon dioxide doped with fluorine is used as a cladding layer in the design. For such a pure-silicon core design, it requires complex viscosity matching inside the fiber, and it is required that extremely low speed is used in a wiredrawing process to avoid an attenuation increase caused by internal defects of the fiber due to high-speed wiredrawing, leading to an extremely complex manufacture process.
EP Patent No. EP2312350 proposes designs of a fiber with a large effective area that does not use a pure-silicon core design. A design of a stepped subsided cladding structure is adapted, and a pure-silicon-dioxide cladding layer structure is used in a design, and related performance can meet requirements of large-effective-area fibers G.654.B and G.654.D. However, in these designs, a maximum radius of a fluorine-doped cladding part is 36 μm. Although it can be ensured that a cable cutoff wavelength of the fiber is less than or equal to 1530 nm, affected by the small fluorine-doped radius, microscopic and macroscopic bending performance of the fiber become worse, and therefore an attenuation increases in a fiber cable process. Related bending performance is also not mentioned in the document.
Chinese Patent No. CN10232392A describes a fiber with a larger effective area. Although an effective area of the fiber in this invention reaches more than 150 μm2, which is realized by adapting a conventional design of a germanium and fluorine co-doped core layer, and by sacrificing the performance index of cutoff wavelength. It allows a cable cutoff wavelength larger than 1450 nm, and in the described embodiments, a cable cutoff wavelength even reaches more than 1800 nm. In an actual application, cutoff in an communication applied waveband in a fiber is difficult to be ensured for an excessively large cutoff wavelength, and therefore a single-mode state in transmission of an optical signal cannot be ensured. Therefore, such a fiber may face a series of practical problems in an application. In addition, in the embodiments listed in this invention, a subsided cladding layer outer-radius r3 is at least 16.3 which is also slightly excessively large. In the disclosure, fiber parameters (such as an effective area and a cutoff wavelength) and fiber manufacture costs are not optimally combined.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.